1. Field of the Invention
The present invention relates generally to catalysts and molecular sieves. In particular, the present invention relates to methods and compositions concerning catalytic and amorphous sulfide sieve materials.
2. Description of Related Art
Molecular sieves are microporous or mesoporous materials with pores of a well-defined, substantially uniform diameter in the range of less than about 20 Å (microporous) and between about 20 and about 500 Å (mesoporous). Most molecules, whether in the gas or liquid phase, both inorganic and organic, have dimensions that fall within either of these ranges for their operating environments. Selecting a molecular sieve with a suitable pore size therefore allows separation of a molecule from a mixture through selective adsorption, hence the name “molecular sieve”. Apart from the selective adsorption and selective separation of uncharged species, the well-defined pore system of a molecular sieve enables selective ion exchange of charged species and selective catalysis. In the latter two cases, significant properties other than the porous structure are, for instance, ion exchange capacity, specific surface area and acidity.
Molecular sieves can be classified in various categories, for example according to their chemical composition or by their structure (e.g., crystalline, amorphous). Traditional molecular sieve materials include crystalline sieve materials such as zeolites and metal silicates. Zeolites are crystalline aluminum silicates with well defined, versatile pore structures that make them useful in a wide variety of applications. Similarly, metal silicates are structurally analogous to zeolites except that they do not contain aluminum or only very small amounts thereof. They are typically based on oxide frameworks.
Metallic sulfides are another type of sieve material. They are a relatively new family of sieves based on sulfide chemistry. An example of early sulfide sieve materials is disclosed in Bedard et al., U.S. Pat. No. 4,880,761. Bedard disclosed a process for preparing a crystalline, metallic sulfide composition having a three-dimensional, micro-porous framework structure of MS2, where M is a metal such as germanium or tin, and S is sulfur (or selenium). Bedard teaches that the sulfide sieve material is formed by reacting a mixture of a templating agent, an anion, a metal sulfide, and water at a temperature, pressure and time sufficient to form the crystalline composition. Bedard teaches that the resulting crystalline three-dimensional microporous composites exhibit adsorption/desorption properties and fluorescence showing that this family of sieves have applications in adsorptive separations, as luminescent display materials, luminescent sensor substrates, and as catalysts, and catalyst supports in metal sulfide-based catalysts such as in hydrogenation, dehydrogenation, dehydration, hydrotreating and conversion reactions.
Other sulfide sieve materials have also recently been developed. (See, e.g., Microporous and Mesoporous Materials, 37 (2000) 243–252). They have typically been close analogs of oxide-based, microporous materials. Most of these newly synthesized microporous sulfide materials have layered structures. For example, SnS is a good semiconductor and is used in chemical sensing devices. Jiang et al (1998) have prepared large single crystals of SnS-n materials. The S-S-1 series A2Sn3S7 (A+=Et4N+, Me4N+, and mixture of NH4+ and Et4N+) has hexagonal-shaped 24-atom rings with diameters of about 10.5 Å in the tin sulfide layer, and interlamellar spacings of 8.5–9.0 Å depending on the templates. (Et4N)2Sn3S7, which has 24-atom rings partially occupied by the template, selectively adsorb H2O over CO2. The SnS-3 series [A2Sn4S9 (A+=Prn4 N+ and Bun4 N+)] has elliptical 32-atom rings of 20×10 Å in size within the tin sulfide layer, and an interlamellar spacing of 14 Å. The tin sulfide trigonal bipyramidal and tetrahedral connecting units make the tin sulfide layer very flexible, and this can undergo elastic deformation to alter the void spaces within and between the layers to accommodate the size and shape of the templates.
Another type of recently developed sulfide sieve material is MoS2 having a large surface area (60 m2/g) and pore size (110 Å in diameter). For example, it has been synthesized via decomposition of (NH4)2Mo3S13xH2O under heating and vacuum treatment (Leist et al., 1998). High-resolution transmission electron microscopy (HRTEM) shows an interlayer spacing of 6–7 Å, and unusually bent lamellae. This bent lamellar structure makes MoS2 an effective lubricant. Driving out the volatile species from the precursor under vacuum assists the MoS2 in forming lamellae with curvature and large pores.
The above-described sieve materials are generally crystalline in structure with atoms and channels that are arranged in complete regularity. Unfortunately, forming such crystalline sieve materials can be difficult and costly in that they typically require relatively expensive precursor solution materials and template removal processes. In addition, they are generally not well suited for reactions with large molecules (e.g., greater than 13 Å).
Another type of sieve materials are amorphous (or noncrystalline) materials. Even though they are not crystalline in structure, they have generally uniform, porous morphologies that allow them to be used in various sieve applications. For example, a group of researchers at Mobil Co. recently reported a series of mesoporous molecular sieves, named the M41S series including MCM-41 and MCM-48, which are noncrystalline (or amorphous) materials. (See U.S. Pat. Nos. 5,057,296 and 5,102,643). These mesoporous molecular sieves have regularly arranged channels larger than those of existing zeolites, thus enabling their application to adsorption, isolation or catalyst conversion reactions of relatively large molecules. The sieves consist of a structure in which mesopores uniform in size are regularly arranged. In contrast to crystalline sieves that have been produced by using inorganic or organic cations as templates, these mesoporous molecular sieves are synthesized through a liquid crystal template pathway by using surfactants as templates. These mesoporous molecular sieves have the advantage that their pore sizes can be adjusted in a range of 16 to 100 Å by controlling the kinds of surfactants or synthetic conditions employed during the production process.
There is a need, however, for new molecular sieves and methods for making such sieves, particularly methods that simplify the process or ones that generate improved molecular sieves.